Antiphospholipid antibodies are associated with an increased risk of venous thrombosis (blood clots in the legs or lungs), arterial thrombosis (especially heart attack or stroke at an early age), or recurrent miscarriages. These antibodies are frequently identified in patients with lupus, but may also be seen in association with certain medications, cancer, other rheumatologic disorders, and during pregnancy. Although most patients with these antibodies have significant clinical problems, up to half of the patients may have no problems at all. Some families have more than one member with these antibodies, and we are interested in defining the genetics of this inherited form of the disease. In order to do this, we are screening patients and their family members for the presence of these antibodies and collecting DNA for genetic analyses. At the same time, we are also characterizing the patients and their family members for other disorders that may impact on their risk of developing blood clots, to better understand the basis of the extreme heterogeneity observed in individuals with these antibodies.